The conditions inside fusion reactors are exceptionally hostile, involving both high temperatures and severe levels of neutron irradiation. All candidate materials for the blanket material are metallic alloys that possess the body-centred cubic (BCC) crystal structure. Alloys with this structure undergo a change from ductile mechanical behaviour at high temperatures to brittle mechanical behaviour (low toughness) at low temperatures. The temperature of this ductile-to-brittle transition (DBT) increases with irradiation, in effect decreasing the fracture toughness of the material and limiting the useful life of components. This increase in DBT temperature is an issue shared by nuclear fission reactors, where it is also life limiting and is the key material parameter in reactor life extension. Typically, the DBT and the transition temperature are measured using macroscopic fracture tests, however, such methods require large volumes of material, and it is usually very difficult to obtain such quantities of material that have undergone irradiation damage. This is an even bigger issue in fusion reactors, where representative irradiated material simply does not exist, and where new alloys will be used, for which no historical test data exists.
For this project I will probe the fundamentals of the ductile-to-brittle transition in BCC alloys using high-resolution digital image correlation (HRDIC), see right-hand image. This technique not only requires very little material, but is able to provide detailed quantitative information about the pattern of deformation inside a material which is, in principle, related to the DBT. The project will test the thesis that HRDIC can be used to measure the DBT in alloys, since the transition involves a change in the deformation behaviour.